REPOET 

OF  THE 

To  the  DIRECTORS  of  the 

NEW  YORK  BRIDGE  COMPANY. 


Avery  Architectural  and  Fine  Arts  Library 
Gift  of  Seymour  B.  Durst  Old  York  Library 


NEW  YORK,  May,  18G9. 


TO  THE  DIRECTORS  OF  THE 


Gentlemen  : 

Having  received  through  Mr.  Roebling,  Engineer  of 
the  New  York  Bridge  Company,  the  request  of  the  Directors  that 
we  should,  as  a  Board  of  Engineers,  make  careful  examination  of 
the  plans  proposed  by  him,  for  the  erection,  over  the  East  River, 
of  a  Wire  Cable  Suspension  Bridge  of  1,600  feet  span  and  135 
feet  elevation  above  low  water,  and  make  report  as  to  the  prac- 
ticability, adequacy,  safety  and  durability  of  the  structure  as  pro- 
posed, and  having  accepted  the  trust  reposed  in  us,  and  having 
given  the  subject  the  full  consideration  and  careful  investigation 
which  its  great  magnitude  and  the  large  interests  involved  de- 
manded, we  have  now  to  submit  a  report  of  our  proceedings  and 
of  the  conclusion  at  which  we  have  arrived. 

On  coming  together  as  a  Board,  our  first  action  was,  to  ob- 
tain a  clear  knowledge  of  the  object  for  which  the  Board  had 
been  convened.  It  was  then  ascertained,  that  the  object  was  not 
one  of  service  to  the  Directors  in  their  consideration  of  the  plans 
proposed,  or  their  action  on  the  adoption  of  the  plans ;  for  that 


4 


consideration  had  been  given,  and  that  action  had  been  taken, 
and  was  before  the  public ;  and  also  that  it  was  not  the  object 
that  the  Engineer  of  the  Company  should  have  the  aid  of  the 
counsel  and  suggestions  of  this  Board,  in  maturing  his  plans,  and 
deciding  the  many  important  questions  involved  in  the  proposed 
structure ;  for  the  plans  had  been  matured,  and  the  decisions  had 
been  presented  in  a  report,  stating  fully  and  clearly  the  general 
character  of  the  structure,  with  such  reference  as  to  details  of 
combination  and  proportions  as  was  appropriate. 

Through  the  inquiries  which  led  to  this  information,  the 
Board  learned,  that  after  the  adoption  of  the  plans  recommended 
by  Mr.  Roebling,  and  after  the  decision  of  the  Directors  to  go 
forward  with  the  undertaking,  Mr.  Roebltng  requested  of  the 
Directors  that  a  Board  of  Engineers  should  be  convened,  before 
whom  the  plans  in  all  their  details  should  be  laid,  in  order  that 
their  professional  judgment  should  be  obtained  as  to  the  prac- 
ticability of  the  erection  of  a  Suspension  Bridge  1,600  feet  span 
and  185  feet  elevation  above  the  water  on  the  plans  proposed  by 
him ;  as  to  the  adequacy  of  the  structure  when  erected  to  sustain 
the  greatest  load  it  would  have  to  carry,  alike  under  the  extremes 
of  summer  and  winter,  and  when  subjected  to  the  action  of  the 
greatest  storms  known  to  take  place.  This  Board  was  informed 
by  Mr.  Roebling,  that  in  view  of  the  magnitude  of  the  under- 
taking and  the  large  interests  connected  therewith,  in  the  cities 
of  New  York  and  Brooklyn,  he  had  suggested  to  the  Directors 
that  it  was  right  and  proper,  before  going  forward  with  the  work, 
that  his  plans  should  be  subjected  to  the  careful  scrutiny  of  a 
Board  of  Engineers  convened  for  the  purpose,  in  order  that  if  his 
plans  received  their  approval,  the  enterprise  would  stand  before 
the  public  sustained  by  their  professional  judgment,  and  would 
not  rest  solely  on  the  judgment  of  one  single  Engineer. 


5 


The  Board  have  undertaken  the  responsible  duty  thus  defined  ; 
a  duty  requiring  the  greater  care,  because  their  action  is  at  the 
request  of  the  Engineer  whose  plans  they  were  to  scrutinize. 

The  plans,  including  foundations,  towers,  and  superstructure, 
have  at  numerous  meetings,  (between  March  25th  and  April  20th,) 
held  for  the  purpose,  been  laid  before  this  Board  by  Mr.  ROEBLING, 
from  whom  they  have  received  at  the  same  time  the  fullest  ex- 
planations on  every  point.  In  the  consideration  of  what  has  thus 
been  brought  before  them,  the  Board  have  been  impressed  by  the 
manner  in  which  Mr.  ROEBLING  has  sought  to  have  an  independ- 
ent investigation  of  his  plans  and  proportions,  of  the  severest 
character. 

Having  completed  the  examination  of  the  plans,  and  the  in- 
vestigations of  the  combinations  and  proportions  proposed  for  the 
service  required,  the  Board  deemed  it  an  appropriate  part  of  their 
duty  to  examine  the  structures  of  the  same  general  character, 
erected  by  Mr.  Roebling,  across  the  Alleghany  and  Pittsburgh 
in  1860,  and  across  the  Niagara  at  Niagara  Falls  in  1854,  and 
across  the  Ohio,  at  Cincinnati,  in  1867.  They  have  thus  had 
under  their  eye  and  personal  examination  the  successive  steps, 
which,  beginning  with  a  span  of  200  in  1844,  was  followed 
by  a  span  of  825  feet  in  1S54,  and  by  a  span  of  1,057  feet  in 
1867,  all  standing  this  day  a  practical  demonstration  of  the 
soundness  of  the  principles  and  proportions  on  which  these 
structures  have  been  erected,  and  rendering  unnecessary,  at  least 
for  spans  of  1,000  feet,  any  other  demonstration,  and  affording 
the  best  source  of  information  as  to  the  practicability  of  taking 
another  step  in  a  span  of  1,600  feet. 

The  report  which,  after  these  investigations,  we  now  concur  in 
making,  is  presented  in  the  following  statements,  having  reference 


6 


therein  to  a  Steel  Wire  Cable  Suspension  Bridge  of  1,600  feet 
between  the  towers,  135  feet  above  the  water,  on  the  plan,  ma- 
terials and  proportion  of  parts  set  forth  in  the  printed  report  of 
Mr.  Roebling,  made  to  the  New  York  Bridge  Company. 

First.  That  the  structure,  when  erected  according  to  the  plans 
and  specifications  proposed  by  Mr.  Roebling,  will  possess  a 
strength  of  parts  not  less  than  six  times  the  strain  to  which  they 
will  be  subjected,  when  under  the  greatest  load  which  will  be  on 
the  Bridge,  and  that  a  strength  of  six  times  the  greatest  strain  is 
a  greater  degree  of  provision  for  safety  than  is  usually  made  in 
structures  having  the  same  object  in  view. 

Second.  That  in  the  combinations  and  relations  of  parts,  as 
proposed  for  the  1,600  feet  span,  there  are  none  of  a  novel  char- 
acter, being  a  repetition  of  what  has  been  done  in  the  Niagara 
Bridge,  and  on  enlarged  proportions  in  the  Cincinnati  Bridge ; 
the  difference  for  the  span  of  1,600  feet  being  a  provision  of  a 
total  strength  to  provide  the  same  degree  of  excess  of  strength  in 
die  greater  span. 

Third.  That  the  combinations  of  parts  provide  in  the  same 
manner  for  the  changes  of  temperature  which  attend  the  ex- 
tremes of  summer  and  winter,  which  has  proved  perfectly  ade- 
quate in  the  Cincinnati  Bridge. 

Fourth.  That  the  structure  is  amply  prepared  to  sustain  the 
action  of  the  greatest  storms  of  which  we  have  any  knowledge. 

Fifth.  That  adequate  foundations  for  the  towers  can  be  had 
in  the  manner  proposed  by  the  Engineer,  the  particular  plan  to 
be  used  depending  on  the  character  of  the  material  developed  in 
the  progress  of  the  work/ ami  that  if  Vood  is  used  in  the  manner 
proposed  under  certain  circumstances,  its  durability  in  such  posi- 
tion is  known  from  the  experience  of  centuries. 


7 


Sixth.  That,  as  to  the  durability  of  iron  put  together  and 
placed  out  of  the  reach  of  all  elements  of  decay,  in  the  manner 
proposed  and  practiced  by  the  Engineer,  no  limit  can  be  assigned  : 
the  facts  being  well  established,  that  iron  not  acted  on  by  oxygen 
is  as  durable  as  any  material  of  which  we  have  knowledge,  and 
that  air  and  water,  the  means  of  furnishing  oxygen,  can  be  readily 
and  with  ordinary  care  permanently  excluded. 

Seventh.  That  the  pressure  on  the  foundations  of  the  towers 
will  not  be  increased  by  the  superstructure,  and  the  greatest  load 
carried  by  the  Bridge  more  than  six  to  seven  per  cent.;  and  that, 
therefore,  the  permanent  stability  of  the  towers  depends  on  the 
principles  well  known  and  used  in  heavy  structures  of  masonry. 

Eighth.  That  there  are  no  peculiar  difficulties  in  erecting 
the  proposed  structure  ;  that  the  same  means  and  arangements  by 
which  the  Suspension  Bridge  of  1,057  feet  span  was  erected  over 
the  Ohio  at  Cincinnati,  are  the  means  and  arrangements  to  be 
made  use  of  in  the  erection  of  a  span  of  1,600  feet  over  the  East 
River. 

Ninth.  That  in  the  use  of  six  trusses  of  large  dimensions  in 
combination  with  the  wire  cable  and  stays,  there  is  provided  all 
the  rigidity  vertically,  lengthwise  the  Bridge,  that  is  required,  and 
that  in  the  roadway  of  80  feet  width,  with  its  horizontal  trusses, 
there  is  secured  a  stability  horizontally  far  exceeding  that  pro- 
vided in  any  previous  structure  of  the  kind. 

To  the  preceding  may  be  added,  that  in  the  inertia  of  the 
mass  comprised  in  the  superstructure  of  a  Suspension  Bridge 
1,600  feet  long  and  80  feet  wide,  there  exists  a  stability  not  to  be 
sensibly  disturbed  by  any  action  which  we  can  anticipate. 

Such  being  the  conclusions  we  have  to  present,  it  will  be  seen 
that  they  sustain  the  following  summary  expression ; 


8 

That  it  is  beyond  doubt  entirely  practicable  to  erect  a  Steel 
Wire  Suspension  Bridge  of  1,G00  feet  span,  135  feet  elevation, 
across  the  East  River,  in  accordance  with  the  plans  proposed  by 
Mr.  Roeblixg,  and  that  such  structure  will  have  all  the  strength, 
stability,  safety  and  durability  that  should  attend  the  permanent 
connection  by  a  Bridge  of  the  cities  of  New  York  and  Brooklyn. 

With  this  expression  of  our  professional  judgment  we  could, 
and  perhaps  should,  close  this  report.  But  in  the  consideration  of 
the  circumstances  that,  in  some  degree,  rendered  advisable  the 
action  of  the  Directors  in  calling  together  this  Board,  we  have 
been  led  to  see  that  it  would  be  in  accordance  with  that  object, 
and  also  be  of  some  practical  value  in  reference  to  the  use  of 
similar  structures  in  other  places  in  our  country,  which  has  yet  so 
much  to  do  in  spanning  its  wide  streams  and  valleys,  to  present, 
in  a  manner  not  requiring  professional  attainment  to  apprehend, 
the  grounds  on  which  confidence  in  the  practicability,  safety 
and  durability  of  structures  of  the  character  proposed  by  Mr. 
Roebling  can  be  attained  to  a  degree  that  will  be  of  practical 
value  to  those  interested  in  structures  requiring  wide  spans. 

The  safety  of  any  structure  to  sustain  strain,  depends  on  the 
relation  of  strength  provided,  to  the  strain  to  be  sustained;  thus,  a 
sound  steel  wire,  about  one-sixth  of  an  inch  in  diameter,  will  sus- 
tain, hanging  on  it  vertically,  4,000  pounds — if  that  weight  be  in- 
creased to  4,100  pounds,  the  wire  will  probably  be  broken.  This 
capacity  of  a  sound  steel  wire  one-sixth  of  an  inch  diameter  to 
sustain  4,000  pounds  hanging  vertically,  is  a  practical  fact,  only 
known  by  actual  numerous  trials  ;  it  is  in  no  degree  a  calculation. 
When  six  wires  of  one-sixth  of  an  inch  diameter  are  put  together 
to  sustain  4,000  pounds,  known  to  be  within  the  capacity  of  a 
single  wire,  it  is  said  that  a  provision  of  six  times  the  strength  is 
made,  and  this  is  what  is  to  be  understood  when  strength  "  by 


9 


calculation"  is  referred  to.  But  when  six  wires  arc  to  be  relied 
on  to  furnish  six  times  the  strength,  it  must  be  known  that  they 
arc  so  combined  that  in  use  the  strain  of  the  weight  is  sustained 
by  the  six  instead  of  the  one.  This  is  matter  of  detail  not  easily 
presented  on  paper  ;  the  manner  of  combination  must  be  seen. 
On  examination  of  the  manner  in  which  the  wires  used  by  Mr. 
Roebling  are  made,  tested  and  combined,  it  will  be  found  that 
they  are  all  straight  wires,  put  together  in  parallel  position,  and 
bound  together  by  temporary  wire  bands,  making  what  is  called 
"  a  strand."  Strands  so  formed  are  put  together  in  parallel  posi- 
tion to  each  other,  and  form  what  is  called  a  wire  cable. 

In  the  use  of  wires  to  sustain  weight  between  towers  wide 
apart,  the  weight  is  not  sustained  by  wires  hanging  vertically,  but 
is  sustained  by  wires  suspended  between,  and  passing  over  the 
towers,  and  held  in  position  by  the  anchorage  on  the  land  side. 

In  such  case,  calculation  and  trial  are  made  use  of  to  ascertain 
what  strain  a  wire  thus  supported  will  be  subjected  to,  when  a 
known  weight  distributed  along  the  wire  is  sustained  with  a  known 
deflection  of  the  wire.  What  that  strain  is,  under  known  circum- 
stances, is  as  fully  and  plainly  a  matter  of  fact,  as  the  strain  is 
known  when  the  wire  hangs  vertically. 

It  is  thus  known  as  plain  matter  of  fact,  that  a  wire  hanging 
between  two  towers  1,600  feet  apart,  with  a  deflection  at  centre 
of  128  feet,  will  sustain  2,300  pounds  beyond  its  own  weight,  and 
that  if  six  wires  so  placed  are  combined  to  sustain  the  2,300  pounds, 
there  will  be  a  provision  of  six  times  the  strain  created  by  the  sus- 
pended weight. 

It  is  also  well  known,  that  the  less  the  depression  of  the  wire 
hanging  between  two  points,  the  greater  the  strain  on  the  wire  ; 
and  that  by  taking  the  proper  depression,  its  own  weight  may  be 


10 


used  to  test  the  strength  and  soundness  of  the  wire,  and  determine 
its  actual  strength.  Mr.  Roebling  makes  use  of  this  means  in 
the  making  up  of  the  strands  of  his  cables. 

When  the  towers  are  ready  to  receive  the  cables,  the  wires 
are  one  at  a  time  stretched  over  the  towers,  and  drawn  by  ade- 
quate power  on  the  land  side  to  a  depression  in  the  centre  of  57 
feet,  the  wire  will  be  subjected  to  a  strain  of  500  pounds.  When 
wires  have  been  so  placed  and  so  proved,  they  are  closely  bound 
together  by  wire  wrappings,  and  thus  is  formed  what  is  called  a 
"  strand,"  each  wire  of  which  is  known  to  have  sustained  a  strain 
of  over  500  pounds.  The  strands  so  formed  and  so  proved  are 
then  lowered  to  a  depression  of  128  feet.  When  the  required 
number  of  strands  are  brought  together,  they  are  tightly  bound 
together  by  wire  wrappings,  using  powerful  clamps  to  bring  the 
strands  closely  in  contact  and  to  the  proper  form,  before  the  wire 
wrapping  is  wound,  round  as  close  together  as  possible.  This 
being  done  from  one  end  to  the  other,  the  wire  cable  is  formed. 
In  each  of  the  four  wire  cables  proposed  for  the  East  River 
Bridge,  19  of  these  strands  are  used,  which  will  swell  their  size 
to  nearly  15  inches  diameter. 

It  is  proposed  to  use  four  steel  wire  cables  of  this  character  in 
the  East  River  Bridge,  every  wire  of  which  will  have  been  proved 
after  it  has  been  stretched  between  the  towers.  It  is  to  be  under- 
stood that  in  this  combination  of  wires  in  a  cable  they  are  brought 
by  powerful  pressure  into  the  closest  contact ;  it  will  then  be  per- 
ceived that  it  would  be  impossible  to  draw  out  a  single  wire  by 
itself;  that  if  adequate  power  was  brought  to  act  on  drawing  out 
a  single  wire  it  would  be  broken,  but  could  not  be  drawn  out. 

Experience  has  shown  that  wire  cables,  although  of  large  di- 
mensions and  great  strength,  are  to  some  degree  flexible,  and  that 


11 


some  means  that  will  give  greater  stiffness  lengthwise  the  Bridge 
is  necessary.  Such  means  are  introduced  by  Mr.  Roebling,  in  the 
Niagara  Bridge,  and  thereby  the  structure  was  made  adequate  to 
the  passage  of  railroad  trains.  They  were  again  used  in  the 
Cincinnati  Bridge.  As  proposed  to  be  used  in  the  East  River 
Bridge,  they  may  be  thus  described  :  They  consist  in  connecting 
with  the  four  wire  cables,  six  trusses  of  iron  12  feet  deep,  which 
arc  to  carry  the  roadway  and  extend  from  tower  to  tower  ;  in  con- 
struction they  are  of  the  kind  now  used  so  extensively  to  form 
railroad  Bridges,  of  up  to  300  to  400  feet  span ;  in  dimensions  of 
parts  they  are  such,  that  any  400  feet  in  length,  supported  at  the 
ends  and  properly  proportioned,  would  more  than  sustain  their  own 
weight  without  aid  from  the  cable  ;  while  the  fact  is,  that  the  truss 
is  supported  by  the  cable  every  7£  feet.  The  use  of  this  combina- 
tion at  Niagara  and  Cincinnati  has  demonstrated  that  the  re- 
quired stiffness  is  thereby  attained. 

In  addition  to  the  use  of  wire  cables  in  combination  with  the 
trusses  as  described,  the  engineer  proposes  to  support  about  570 
feet  of  each  end  of  the  Bridge  next  the  towers  by  wire  ropes,  of 
2$  inches  diameter,  which  being  connected  with  the  towers  near 
the  top  and  with  the  road  bed  at  distances  from  the  tower  of  50 
feet  up  to  570  feet  of  the  roadway,  in  a  manner  well  known  and 
extensively  used. 

The  strength  of  each  of  these  wire  ropes,  called,  from  their  posi- 
tion and  supporting  action  on  the  roadway,  Hays,  is  practically 
known,  and  also  their  sustaining  power,  brought  to  bear  on  the 
first  five  hundred  feet  from  the  tower. 

From  the  familiar  description  of  the  East  River  Bridge  as  pro- 
posed by  Mr.  Roebling,  it  will  be  seen  that  the  structure  will  be 
a  combination  of  four  steel  wire  cables,  of  near  15  inches  diameter, 


12 


with  six  iron  trusses  extending  from  tower  to  tower,  and  128 
stays  attached  to  each  tower ;  a  combination  which  in  principle 
has  been  adequately  proved  at  Niagara  and  Cincinnati,  and  which 
provides  as  matter  of  fact  a  strength  equal  to  six  times  the  strain 
that  will  result  from  the  greatest  load  that  will  ever  be  on  the 
structure. 

As  to  the  adequacy  of  the  towers  to  perform  their  part,  it  is 
to  be  understood  that  the  pressure  on  the  towers  due  to  the  weight 
of  the  wire  cables,  trusses,  roadway  etc.,  act  vertically  through  the 
tower,  because  the  wire  cable,  lying  in  the  top  of  the  towers,  ex- 
tends on  both  sides  at  about  the  same  angle  of  depression,  and  is 
on  the  land  side  held  by  a  mass  of  masonry  whose  total  weight 
exceeds  largely  the  tension  of  the  cables. 

To  this  may  be  added,  that  a  force  equal  to  about  4,000  tons, 
acting  horizontally  on  the  top  of  the  tower  in  the  direction  of  the 
Bridge,  would  be  required  to  upset  the  tower  when  held  in  posi- 
tion only  by  its  own  weight,  and  constructed  as  a  solid  mass. 

As  to  durability.  The  durability  of  any  material  or  structure 
depends  on  excluding  all  causes  tending  to  change  the  condition 
of  the  material  or  of  the  combination  of  the  structure. 

A  structure  of  masonry  in  this  climate,  from  which  water  was 
not  excluded  by  proper  means,  would  have  less  durability  than 
one  of  iron  under  same  exposure.  The  action  of  cold  in  the  win- 
ter, if  water  was  allowed  to  be  present,  would  soon  show  that  care 
must  be  taken  to  secure  the  desired  durability. 

The  known  durability  of  wood,  when  used  in  foundations  at 
great  depths  and  always  saturated  with  water,  finds  its  explana- 
tion in  the  fact  that  there  are  no  changes  in  the  condition. 
There  are  no  alternations  of  wet  and  dry,  and  no  material  changes 


13 


of  temperature,  and  a  long  experience  has  proved  that  under  such 
circumstances  the  condition  of  the  material  remains  the  same. 

In  the  use  of  iron  and  steel,  the  same  kind  of  care  must  be 
taken.  Change  in  the  condition  of  iron  or  steel  can  only  take 
place  from  rusting ;  that  is,  from  combination  with  oxygen. 
Oxygen  can  only  have  access  to  the  iron  or  steel  from  water  or 
air.  If  water  and  air  are  excluded  entirely  from  contact  with 
the  iron  or  steel,  there  can  be  no  oxidation,  and  therefore  no 
change  in  the  condition,  consequently  no  change  in  strength. 

That  it  may  be  understood  in  what  way  and  by  what  means 
water  and  air  are  to  be  permanently  excluded,  it  is  to  be  stated, 
that  when  the  strands,  containing  about  270  parallel  straight  wires, 
are  made  up,  all  the  interstices  between  the  wires  are  thoroughly 
filled  with  pure  boiled  linseed  oil,  and  that  when  the  cable  is 
made  up  from  19  of  these  strands,  oil  is  introduced  so  thoroughly, 
that  when  the  cable  is  wound,  there  is  not  an  interstice  between 
the  wires  that  is  not  filled  with  oil.  The  winding  of  the  cable 
thus  formed  and  saturated  with  oil,  is  by  means  of  wire  one-tenth 
of  an  inch  diameter,  which  is  wound  so  close  together  on  the 
exterior  of  the  cable  as  to  form  a  surface  nearly  impervious  to 
water;  that  surface  of  wire  is  then  painted  with  pure  oil  in  several 
coats. 

It  will  be  readily  understood,  that  as  long  as  the  external 
painting  is  properly  renewed  from  time  to  time,  neither  water  nor 
air  can  ever  have  access  to  the  wires,  and  as  long  as  this  state  of 
things  continues,  the  unchanged  condition  of  the  wire  must  con- 
tinue, and  consequently  the  strength  of  the  material  must  continue 
unimpaired. 


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